The invention relates to a fluid stabilized plasma generator arrangement utilized to apply coatings to substrates. More specifically, the invention relates to a semi-transferred arc stabilized plasma generator utilized to apply coatings to substrates and method for utilizing said plasma generator.
Liquid stabilized plasma generators have been utilized to apply coatings to substrates. U.S. Pat. No. 4,338,509 issued July 6, 1982 for a "Process of and Apparatus for Producing a Homogeneous Radially Confined Plasma Stream" shows a such a liquid stabilized plasma generator. In an apparatus such as that illustrated in U.S. Pat. No. 4,338,509, the electric arc is struck between the tip of the rod cathode and the edge of the rotating anode. The arc does not extend past the anode toward the substrate. Stabilizing liquid, such as water, entering a stabilizing chamber in the plasma generator encounters the arc and disassociates into its plasma state. The plasma then passes from the chamber as a plasma stream through a nozzle orifice. Coating material in particulate form is fed into the plasma stream downstream of the anode. The coating material is transformed into its liquid, gaseous and/or plasma states depending upon the energy of the stream and the particular coating material.
The plasma stream extends past the nozzle a certain distance at which it may be considered to terminate. The charged particles in the plasma stream past the anode recombine so as to release thermal energy. This release of thermal energy upon recombination is what provides the plasma stream with its very high temperature. Practically speaking, the charged particles have all recombined into certain compounds past the termination point or end of the plasma stream. These compounds are then carried on to the substrate so as to apply a coating, which is comprised of these compounds, to the substrate.
In an arrangement such as that illustrated in U.S. Pat. No. 4,338,509, if the surface of the substrate is past (or downstream of) the termination of the plasma stream, there is essentially no concentration of charged particles at the surface of the substrate. In this condition, the surface is not "active" or prone to chemically react with the coating compounds which, prior to impingement on the surface, have already recombined. The result is that a physical-type bond is formed between the substrate and the coating.
In some coating applications, a coating which is physically bonded to the substrate is satisfactory. However, in many coating applications, it would be desirable to provide a coating that is chemically bonded to the substrate.
As is apparent from the above discussion, the temperature of the substrate and of the coating material as it is deposited on the substrate are two parameters that must be considered if one desires to form a chemical bond between the coating and the substrate. Furthermore, if some of the coating material is ionized at the surface of the substrate, chemical bonding is more likely. The tendency of the substrate surface material to chemically react with the coating material increases as the temperature of the substrate and coating material increases. It would thus be desirable to provide a semi-transferred liquid stabilized plasma generator wherein the surface of the substrate could be raised to a temperature that would enhance its ability or tendency to chemically react with the coating material.
Another parameter which must be considered is the velocity at which the coating material impinges upon the surface of the substrate. The plasma generator illustrated in U.S. Pat. No. 4,338,509 operates so that the coating material impinges the substrate at a sufficient velocity to form a bond between the substrate and the coating. However, this bond is a physical-type of bond. If one desires to form a chemical-type bond between the substrate and the coating, it would be desirable if the velocity of the coating material impinging on the surface of the substrate would be higher than that velocity capable of being produced by the operation of the non-transferred arc liquid stabilized plasma generator illustrated in U.S. Pat. No. 4,338,509.
In some coating applications, it is desirable to form certain compounds such as, for example, oxides, carbides and nitrides as compounds in the coating. However, the direct feeding of particulate carbide material or the like into the plasma stream may create difficulties with the optimum application of a coating. Thus, in some of the above applications it would be desirable if the stabilizing liquid would be of a chemical composition to include the necessary compound-forming elements, such as oxygen, carbon and nitrogen atoms required to form the oxides, carbides and nitrides. However, if the temperature of the substrate surface is not sufficiently high and the velocity of the plasma stream is not sufficiently high, then there is not the optimum formation of compounds such as the oxides, carbides or nitrides. It would thus be desirable to provide a semi-transferred arc liquid stabilized plasma generator arrangement wherein operating parameters are provided for the optimum formation of compounds in the coatings, for example, oxides, nitrides or carbides, wherein at least some of the compound forming elements such as atoms, free electrons and ions are constituents of the stabilizing liquid.
In some coating applications, it is desirable that certain compounds be formed in the coating wherein the compound forming elements are contained in the anode of the plasma generator arrangement. In other words, the anode may be comprised of materials such as iron, copper, aluminum or graphite wherein it is desired that one of those elements be a constituent of a compound in the coating. It would thus be desirable to provide a semi-transferred arc liquid stabilized plasma generator arrangement wherein operating parameters are provided for the optimum formation of compounds in the coating wherein the compound forming elements, such as atoms, free electrons and ions, are constituents of the anode.
In certain applications, it would be desirable to form a coating on the substrate that includes non-stoichiometric compounds or compounds that are chemical intermediates in the chemical reactions that would normally occur at a generally constant temperature. By varying the temperature of chemical reaction, they can sometimes be controlled to produce chemical intermediates. Thus, it would be desirable to provide a semi-transferred arc liquid stabilized plasma generator that can control or vary the temperature at the substrate surface.
In certain coating applications, it would be desirable if the surface of the substrate was very prone to chemically react with the coating material. This could be enhanced if the surface was depleted of electrons so as to leave charged ions at the surface. These ions would be very prone to react with the coating material. Thus, it would be desirable to provide a semi-transferred arc liquid stabilized plasma generator that is arranged so as to cause an electron migration from the surface of the substrate.
A deficiency in the prior art is the lack of a process and apparatus for varying the reactivity of the plasma stream with the substrate and with the coating material during the time that the coating material is being deposited on the substrate.